1. Field of the invention
The present invention relates to industrial polyester fibers useful as reinforcements for rubber articles such as tires and belts. More particularly, the present invention relates to industrial polyester fibers, which are of high tenacity as well as of high modulus and low shrinkage, useful for the production of dipped cords (D/C) with superior dimensional stability. Also, the pre sent invention is concerned with a method for preparing such an industrial polyester fiber.
2. Description of the Prior Art
At present, widely used as reinforcements for rubber articles, such as tires and belts, are HMLS (high modulus low shrinkage) dipped cords which range, in E-S (intermediate elongation+shrinkage), from 6.5 to 8.0% with high dimensional stability. Generally, the grey yarns for these dipped cords are prepared by extruding molten polyester polymers, winding the extruded polyester polymers at a speed of 2,000 m/min or higher to produce a birefringence of at least 40.times.10.sup.-3 in the resulting undrawn yarns, and drawing the undrawn yarns with the aid of a godet roller.
Since the first introduction of polyester grey fibers, characteristic of low shrinkage, low work loss, and high tenacity, in U.S. Pat. No. 4,101,525 (Davis, et al), references directed to industrial HMLS polyester fibers are found in many patents.
U.S. Pat. No. 4,491,657, for example, discloses a polyester multifilament yarn which has high modulus and low shrinkage and is useful in the textile reinforcement of tires, asserting that an improvement in tenacity can be brought about in twisted yarns and dipped cords when HMLS polyester multifilament yarns have a low terminal modulus. Since such a low terminal modulus demands lowering the draw ratio, it is needed to control the terminal modulus to a level in which the high tenacity of grey yarns can be appropriately expressed in order to produce high tenacity dipped cords.
In order to make industrial polyester grey yarns of high tenacity, conventionally, a heater is used to heat the temperature just below a spinning nozzle to lower the undrawn orientation of the yarns, followed by drawing at a high draw ratio. Alternatively, the grey yarns are spun at a low speed to lower the undrawn orientation and then, drawn at a high draw ratio. These conventional methods, however, cause an increase in shrinkage index, deteriorating the dimensional stability of final dipped cords. In addition, these conventional methods suffer from a disadvantage in that the yarns obtained are lowered in tenacity after undergoing the thermal treatment for twisted yarns.
Another reference concerning industrial HMLS polyester fibers is found in U.S. Pat. No. 4,690,866, which discloses that polyester chips with an intrinsic viscosity of at least 1.2 are spun to produce high tenacity HMLS grey yarns. Where the intrinsic viscosity of chips is increased, an increase occurs in the spinning tension of yarns and thus also in their orientation, giving contribution to the dimensional stability of the resulting dipped cords. In a view of the molecular structure of grey yarns, tie chains, which connect non-crystalline moieties with crystalline moieties, are formed at a large amount owing to the increase in the intrinsic viscosity of chips, so that the grey yarns show high tenacity even after being drawn at low draw ratios. The increase in the intrinsic viscosity of polyester chips is usually achieved by solid-polymerization.
Upon the solid-polymerization, however, the difference in intrinsic viscosity between chip surface and chip center is more serious as the viscosity becomes higher. Thus, the resulting inhomogeneous viscosity over the polymer incurs a decrease in spinning property and requires high temperatures upon melt-spinning. In turn, the high melting temperature promotes thermal decomposition and hydrolysis in the polymer. Therefore, the intrinsic viscosity of the filaments spun is not increased to the extent to which the viscosity of chips is increased. In practice, the intrinsic viscosity of chips is increased over the theoretical values in order for the yarns to obtain a desired viscosity. Particularly, the inhomogeneity of viscosity over the polymer causes yarns to frequently undergo filament cutting, making the appearance of and processability for yarns poor. In result, this conventional technique is economically disadvantageous in terms of time and energy.
In producing HMLS fibers, oiling is usually conducted to provide smoothness and packability for grey yarns. Oiling agents used for the oiling process are largely divided into two types: non-aqueous oiling agents and aqueous oiling agents. For non-aqueous oiling agents, crude oiling agents are mixed with mineral oil agents (straight oil) or used as they are (neat-oil). Non-aqueous oiling agents assure good processability, but are insufficient in safety, for example, apt to catch fire during processing. In addition, non-aqueous oiling agents are economically unfavorable in that they require additional heating means to maintain their appropriate viscosity and are expensive. On the other hand, aqueous oiling agents are good in safety and economic aspects compared with the non-aqueous oiling agents, but problematic in processability. Particularly, aqueous oiling agents are not suitable for high speed, direct spin-drawing processes. Therefore, there remains a need for an aqueous oiling agent which can be used for high speed, direct spin-drawing processes without complications.